The present invention relates to a method and electronic device for measuring the electric power of a universal electric motor powered by sinusoidal alternating voltage.
To reduce production cost, some electric household appliances, such as washing machines, are equipped by the manufacturers with so-called “universal” electric motors, which have the big advantage of being powered with alternating current by particularly simple, low-cost power circuits employing Triacs, while at the same time providing similar performance as direct-current electric motors, i.e. high starting torque, compactness, high rotation speed, etc.
It should be pointed out that the term “universal motor” is used herein to indicate a wire-wound stator motor that can operate on both direct and alternating current. Alternating-current operation of the motor is made possible by the fact that, thanks to the power circuit, the electric currents in the stator and rotor of the motor are inverted simultaneously, and the relative magnetic fields work in the same way, generating the same type of attraction or repulsion force.
As is known, certain operating parameters of the appliance must be measured to permit a direct electronic control of the appliance working. More specifically, the latest washing machines normally comprise a control device that controls the electric motor by regulating its speed instant by instant to counteract, for example, any unbalance caused by the laundry in the laundry drum during the spin cycle, and so reduce walking and/or the noise level of the appliance.
Processing by the control device is substantially based on determining mechanical quantities, such as the speed and torque of the motor. More specifically, the speed of the motor is determined “directly” by a speed sensor fitted to the drive shaft, whereas mechanical torque is determined “indirectly” by processing electric parameters, such as the mean electric power of the motor.
To calculate electric power, the methods employed by the computing units of many control devices substantially comprise a complete reconstruction of the time patterns of current and voltage supply to the motor at each period, and, though determining mean electric power fairly accurately, have the drawback of requiring particularly high-computing-power units with high frequency sampling rate capability, which are complex and expensive and so greatly increase the overall production cost of the appliance.